U.S. Patent of U.S. Pat. No. 6,417,752 discloses a stick-type ignition coil device whose peripheral core is exposed to a plug hole. FIG. 4 shows an exploded perspective view of the coil ignition device 100. As shown in FIG. 4, the ignition coil device 100 includes a secondary spool 101, a primary spool 102, and a peripheral core 103. The ignition coil device 100 is inserted in a plug hole (not shown). The secondary spool 101 is cylindrical. A secondary coil wire (not shown) is wound around an outer surface of the secondary spool 101. The primary spool 102 is cylindrical. The primary spool 102 is disposed as surrounding the secondary coil wire. A primary coil wire (not shown) is wound around an outer surface of the primary spool 102. The peripheral core 103 is cylindrical with having a slit 104 longitudinally extending. The peripheral core 103 is disposed as surrounding the primary coil wire with being exposed within the plug hole.
Non-crystalline resin such as PPE (Polyphenylene ether) sometimes develops cracks due to an even slight stress after contacting given gas, liquid, or solid. It is because developing of structure change such as breakage or cross bridging of molecular chains results in lowering strength. These phenomena are called ESC (environment stress crack). An instance of combinations of substances developing ESC is a combination of non-crystalline resin and blowby gas that is mixed gas including combustion gas, non-combustion gas, and atomized engine oil from an engine combustion chamber.
In the plug hole, the blowby gas flows from the engine combustion chamber through a plug insertion hole disposed in the bottom of the plug hole. In the ignition plug device 100, the primary spool 102 is exposed to the blowby gas that flows in through the slit 104 of the peripheral core 103 and then space between turns of the primary coil wire.
The primary spool 102 is generally formed of non-crystalline resin that has high adhesiveness to epoxy resin (not shown) filled in the ignition coil device 100. Furthermore, linear expansion coefficients of the primary spool 102 and members around the primary spool 102 are different. The primary spool 102 thereby suffers thermal stress due to heating/cooling cycles of an engine.
Thus, the primary spool 102 exposed to the blowby gas for a long time frame suffers the thermal stress, so that the primary spool 102 has possibility of developing the ESC. As a result, the ignition coil device 100 has poor environment resistance.